M APPING I NDIA ’ S MAGNETIC FIELD Because of its practical application in prospecting, communication, and navigation, the magnetic field of the earth is mapped by most nations with an accuracy comparable to geographical mapping. In India over a dozen observatories exist, extending from Trivandrum (now Thrivuvananthapuram) in the south to Gulmarg in the north. These observatories work under the aegis of the Indian Institute of Geomagnetism (IIG), in Colaba, Mumbai. The IIG grew out of the Colaba and Alibag observatories and was formally established in .
The IIG monitors (via its nation-wide observatories), the geomagnetic fields and fluctuations on land, and under the ocean and in space. Its services are used by the Oil and Natural Gas Corporation Ltd. (ONGC), the National Institute of Oceanography (NIO) and the Indian Space Research Organisation (ISRO). It is a part of the world-wide network which ceaselessly updates the geomagnetic data.
Now India has a permanent station called Gangotri. . Three quantities are needed to specify the magnetic field of the earth on its surface – the horizontal component, the magnetic declination, and the magnetic dip. These are known as the elements of the earth’s magnetic field.
. Consider a material placed in an external magnetic field B . The magnetic intensity is defined as, µ = B H The magnetisation M of the material is its dipole moment per unit volume. The magnetic field B in the material is, B = µ ( H + M ) .
For a linear material M = χ H . So that B = µ H and χ is called the magnetic susceptibility of the material. The three quantities, χ , the relative magnetic permeability µ r , and the magnetic permeability µ are related as follows: µ = µ µ r µ r = + χ . Magnetic materials are broadly classified as: diamagnetic, paramagnetic, and ferromagnetic.
For diamagnetic materials χ is negative and small and for paramagnetic materials it is positive and small. Ferromagnetic materials have large χ and are characterised by non-linear relation between B and H . They show the property of hysteresis. .
Substances, which at room temperature, retain their ferromagnetic property for a long period of time are called permanent magnets. Physical quantity Symbol Nature Dimensions Units Remarks Permeability of µ Scalar [MLT – A – ] T m A – µ / π = – free space Magnetic field, B Vector [MT – A – ] T (tesla) G (gauss) = T Magnetic induction, Magnetic flux density Magnetic moment Vector [L – A] A m Magnetic flux φ B Scalar [ML T – A – ] W (weber) W = T m Magnetisation M Vector [L – A] A m – Magnetic moment Volume Magnetic intensity H Vector [L – A] A m – B = µ ( H + M ) Magnetic field strength Magnetic χ Scalar - - M = χ H susceptibility Relative magnetic µ r Scalar - - B = µ µ r H permeability Magnetic permeability µ Scalar [MLT – A – ] T m A – µ = µ µ r N A – B = µ H